Amine oxide coating compositions

ABSTRACT

An aqueous coating composition is provided comprising carbon particles, one or more highly fluorinated polymers and one or more amine oxide surfactants, typically an amine oxide surfactant according to formula I:  
                 
 
     where n is 9 to 19. The coating composition is advantageously used in a method of making a gas diffusion layer for an electrochemical cell comprising the steps of: a) coating a electrically conductive porous substrate with the aqueous coating composition, and b) heating the electrically conductive porous substrate to a temperature sufficient to decompose the amine oxide surfactants. In another aspect, a gas diffusion layer for an electrochemical cell is provided which is advantageously free of surfactant.

[0001] This invention was made with Government support under CooperativeAgreement DE-FC02-99EE50582 awarded by DOE. The Government has certainrights in this invention.

FIELD OF THE INVENTION

[0002] This invention relates to an aqueous coating composition usefulfor making a gas diffusion layer for an electrochemical cell such as afuel cell, which layer is advantageously free of surfactant residue. Thecoating composition comprises carbon particles, one or more highlyfluorinated polymers, and one or more amine oxide surfactants.

BACKGROUND OF THE INVENTION

[0003] U.S. Pat. No. 6,127,059 describes the use of Triton™ X100, analcohol alkoxylate, as a surfactant when coating a gas diffusion layerfor use in an electrochemical cell with a fluororesin containing carbonblack.

SUMMARY OF THE INVENTION

[0004] Briefly, the present invention provides an aqueous coatingcomposition comprising carbon particles, one or more highly fluorinatedpolymers and one or more amine oxide surfactants, typically an amineoxide surfactant according to formula I:

[0005] where n is 9 to 19.

[0006] In another aspect, the present invention provides a method ofmaking a gas diffusion layer for an electrochemical cell comprising thesteps of: a) coating a electrically conductive porous substrate with theaqueous coating composition according to the present invention, and b)heating the electrically conductive porous substrate to a temperaturesufficient to decompose the amine oxide surfactants.

[0007] In another aspect, the present invention provides a gas diffusionlayer for an electrochemical cell made according to the method of thepresent invention which typically contains no surfactant.

[0008] What has not been described in the art, and is provided by thepresent invention, is a coating composition and method of using samewhich contains a decomposing amine oxide surfactant and can thereforeprovide a surfactant-free gas diffusion layer.

[0009] In this application:

[0010] “vehicle” means a fluid which carries the particulate in adispersion, which typically includes water or an alcohol;

[0011] “highly fluorinated” means containing fluorine in an amount of 40wt % or more, but typically 50 wt % or more, and more typically 60 wt %or more; and

[0012] “substituted” means, for a chemical species, substituted byconventional substituents which do not interfere with the desiredproduct or process, e.g., substituents can be alkyl, alkoxy, aryl,phenyl, halo (F, Cl, Br, I), cyano, nitro, etc.

[0013] It is an advantage of the present invention to provide a coatingcomposition and method of using same which contains an effectivesurfactant which is ephemeral in use, and therefore capable of providinga surfactant-free gas diffusion layer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0014] The present invention provides an aqueous coating compositioncomprising carbon particles, one or more highly fluorinated polymers andone or more amine oxide surfactants.

[0015] Fuel cells are electrochemical cells which produce usableelectricity by the catalyzed combination of a fuel such as hydrogen andan oxidant such as oxygen. Typical fuel cells contain layers known asgas diffusion layers or diffuser/current collector layers adjacent tocatalytically reactive sites. These layers must be electricallyconductive yet must be able to allow the passage of reactant and productfluids. Typical gas diffusion layers are coated with a layer of carbonparticles and fluoropolymers on the surface adjacent to the catalyst.

[0016] The coating composition may employ any suitable aqueous vehicle.The vehicle comprises water and may additionally comprise alcohols, andmore typically comprises only water or alcohols. Most typically thevehicle comprises water alone.

[0017] Any suitable amine oxide surfactants may be used. Suitable amineoxides may belong to formula II: R₃N→O, where each R is independentlyselected from alkyl groups containing 1-20 carbons, which optionallyinclude ether and alcohol groups, and which may be additionallysubstituted. Typically the amine oxide surfactant is an Us amine oxideaccording to formula I:

[0018] where n is 9 to 19 or more typically 11-15. Most typically, n is11 or 13. The amine oxide according to formula I is optionallysubstituted. Suitable amine oxide surfactants may include thoseavailable under the trade names Genaminox®, Admox®, Ammonyx®, andNinox®. The amine oxide surfactant advantageously has a decompositiontemperature of less than 200° C., more typically less than 180° C. andmost typically less than 160° C. The aqueous coating compositiontypically comprises 0.1- 15% amine oxide surfactant by weight, moretypically 0.1-10% by weight, and most typically 1-5% by weight.

[0019] Any suitable carbon particles may be used. It will be understoodthat the term “carbon particles” as used herein can refer to primaryparticles, typically having a average size of 1-100 nm, primaryaggregates of primary particles, typically having an average size of0.01-1 microns, secondary aggregates of primary aggregates, typicallyhaving an average size of 0.1-10 microns, and agglomerates ofaggregates, typically having an average size of greater than 10 micron.Most typically, the term “carbon particles” refers to primary particlesor primary aggregates. Typically a carbon black is used, such as VulcanXC-72 (Cabot Corp., Special Blacks Division, Billerica, Mass.),Shawinigan Black, grade C55, (Chevron Phillips Chemical Company, LP,Acetylene Black Unit, Baytown, Tex.) or Ketjenblack EC300J (Akzo NobelChemicals Inc., Chicago, Ill.). Graphite particles may also be used, buttypically have larger particle sizes. The aqueous coating compositiontypically comprises 1-50% carbon particles by weight, more typically1-20% by weight, and most typically 5-15% by weight. Typically, theaqueous coating composition comprises lower weight percent content ofcarbon particles where smaller particles are used. The highest weightpercent content of carbon particles are achieved with the addition ofgraphite particles, which typically have larger particle sizes.

[0020] Any suitable highly fluorinated polymers may be used. The highlyfluorinated polymer is typically a perfluorinated polymer, such aspolytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP),perfluoroalkyl acrylates, hexafluoropropylene copolymers,tetrafluoroethylene/hexafluoropropylene/vinylidene fluoride terpolymers,and the like. The aqueous coating composition typically comprises0.1-15% highly fluorinated polymers by weight, more typically 0.1-10% byweight, and most typically 1-5% by weight.

[0021] In the method according to the present invention, a gas diffusionlayer for an electrochemical cell is made by the steps of: a) coating aelectrically conductive porous substrate with the aqueous coatingcomposition described above, and b) heating the electrically conductiveporous substrate to a temperature sufficient to decompose the amineoxide surfactants.

[0022] Any suitable electrically conductive porous substrate may beused. Typically, the electrically conductive porous substrate is acarbon fiber construction. Typically carbon fiber constructions areselected from woven and non-woven carbon fiber constructions. Carbonfiber constructions which may be useful in the practice of the presentinvention may include: Toray™ Carbon Paper, SpectraCarb™ Carbon Paper,AFN™ non-woven carbon cloth, Zoltek™ Carbon Cloth, and the like. Theporous substrate may be treated prior to coating. Typical treatmentsinclude those that increase or impart hydrophobic properties, such astreatment with fluoropolymers such as PTFE. Other typical treatments mayincrease or impart hydrophilic properties.

[0023] Any suitable method of coating may be used. Typical methodsinclude both hand and machine methods, including hand brushing, notchbar coating, wire-wound rod coating, fluid bearing coating, slot-fedknife coating, and three-roll coating. Coating may be achieved in onepass or in multiple passes.

[0024] The heating step may be accomplished by any suitable method.Advantageously, the temperature and duration of the heating step issufficient to decompose all of the surfactant. Advantageously, thetemperature and duration of the heating step is sufficient to decomposeand remove all of the surfactant and decomposition products of thesurfactant. In addition, the temperature and duration of the heatingstep is advantageously sufficient to sinter the fluoropolymer. In oneembodiment, the decomposition temperature is at least 5° C. less thanthe sintering temperature of the highly fluorinated polymers. In thisway, the surfactant can be substantially removed before sintering of thefluoropolymers on the substrate. More typically, the decompositiontemperature is at least 10° C. less than the sintering temperature. Mosttypically, the decomposition temperature is at least 20° C. less thanthe sintering temperature.

[0025] The present invention advantageously provides a gas diffusionlayer for an electrochemical cell which contains substantially nosurfactant. Typically, the a gas diffusion layer according to thepresent invention contains substantially no surfactant or surfactantdecomposition products.

[0026] The resulting gas diffusion layer is typically incorporated intoa membrane electrode assembly for use in an electrochemical cell such asa hydrogen fuel cell by any suitable method, many of which are known inthe art.

[0027] This invention is useful in the manufacture of a gas diffusionlayer for use in electrochemical cells such as hydrogen fuel cells.

[0028] Various modifications and alterations of this invention willbecome apparent to those skilled in the art without departing from thescope and principles of this invention, and it should be understood thatthis invention is not to be unduly limited to the illustrativeembodiments set forth hereinabove. All publications and patents areherein incorporated by reference to the same extent as if eachindividual publication or patent was specifically and individuallyindicated to be incorporated by reference.

We claim:
 1. An aqueous coating composition comprising carbon particles,one or more highly fluorinated polymers and one or more amine oxidesurfactants.
 2. The composition according to claim 1 wherein said amineoxide surfactants have a decomposition temperature of less than 200° C.3. The composition according to claim 1 wherein said amine oxidesurfactants are alkyl dimethylamine oxides according to formula II:R₃N→O  (II) where each R is independently selected from alkyl groupscontaining 1-20 carbons which may include ether oxygens, which may besubstituted with alcohol groups, and which may be additionallysubstituted.
 4. The composition according to claim 3 wherein said amineoxide surfactants have a decomposition temperature of less than 200° C.5. The composition according to claim 1 wherein said amine oxidesurfactants are alkyl dimethylamine oxides according to formula I:

wherein n is 9 to 19, and which is optionally substituted.
 6. Thecomposition according to claim 5 wherein said amine oxide surfactantshave a decomposition temperature of less than 200° C.
 7. The compositionaccording to claim 5 wherein n is 11 to
 15. 8. The composition accordingto claim 7 comprising: 1-50% by weight carbon particles; 0.1-15% byweight highly fluorinated polymers; and 0.1-15% by weight amine oxidesurfactant.
 9. The composition of claim 1 wherein said highlyfluorinated polymers have a sintering temperature and wherein said amineoxide surfactants have a decomposition temperature which is lower thansaid sintering temperature by at least 5° C.
 10. A method of making agas diffusion layer for an electrochemical cell comprising the steps of:a) coating a electrically conductive porous substrate with the aqueouscoating composition according to claim 1; and b) heating theelectrically conductive porous substrate to a temperature sufficient todecompose said amine oxide surfactants.
 11. The method according toclaim 10 wherein said electrically conductive porous substrate comprisescarbon fiber.
 12. The method according to claim 10 wherein said highlyfluorinated polymers have a sintering temperature and wherein said amineoxide surfactants have a decomposition temperature which is lower thansaid sintering temperature by at least 5° C.
 13. The method according toclaim 10 wherein said step of heating the electrically conductive poroussubstrate comprises heating the electrically conductive porous substrateto a temperature sufficient to sinter said highly fluorinated polymers.14. The method according to claim 12 wherein said step of heating theelectrically conductive porous substrate comprises heating theelectrically conductive porous substrate to a temperature sufficient tosinter said highly fluorinated polymers.
 15. A gas diffusion layer foran electrochemical cell made according to the method of claim
 10. 16. Agas diffusion layer for an electrochemical cell according to claim 15which comprises essentially no surfactant.
 17. A membrane electrodeassembly for an electrochemical cell comprising the gas diffusion layeraccording to claim
 15. 18. A membrane electrode assembly for anelectrochemical cell comprising the gas diffusion layer according toclaim 16.